This disclosure describes means to provide Sagnac sensing systems similar to those described in detail in U.S. Pat. No. 4,898,468 (E. Udd, Sagnac Distributed Sensor, Feb. 6, 1990), U.S. Pat. No. 4,976507 (E. Udd, Sagnac Distributed Sensor, Dec. 11, 1990), U.S. Pat. No. 5,046,848 (E. Udd, Fiber Optic Detection System Using a Sagnac Interferometer, Sep. 10, 1991), U.S. Pat. No. 5,402,231 (E. Udd, Distributed Sagnac Sensor Systems, Mar. 28, 1995), and U.S. Pat. No. 5,636,021 (E. Udd, Sagnac/Michelson Distributed Sensing Systems, Jun. 3, 1997) using a single installed optical fiber. Early work on using the Sagnac interferometer to detect time varying events can be found in U.S. Pat. No. 4,375,680 by Richard Cahill and Eric Udd, xe2x80x9cOptical Acoustic Sensorxe2x80x9d issued Mar. 1, 1983. The teachings in those patents are incorporated into this disclosure by reference as though fully set forth below.
This invention relates generally to fiber optic sensing and more particularly to use of the Sagnac interferometer to measure and locate a time varying event. In order to make a system of this type more practical and widely applicable for general use, a flexible, single fiber configuration is needed to support the base of currently installed optical fiber and to simplify future installations.
The Sagnac interferometer provides means to sense time varying events such as acoustic waves and vibrations with high sensitivity and unique optical filtering action as is described by Eric Udd in xe2x80x9cFiber Optic Sensors Based on the Sagnac Interferometer and Passive Ring Resonatorxe2x80x9d, Fiber Optic Sensors: An Introduction for Engineers and Scientists, E. Udd Editor, Wiley, 1991. These properties in combination with the ability of the Sagnac interferometer to be supported by low cost components, such as light emitting diodes, have allowed the usage of these devices as optical microphones, hydrophones and for intrusion sensing.
By combining properties of the Sagnac interferometers with color coded reflectors such as fiber gratings, it is possible to measure the presence of a time varying signal and localize it on a single optical fiber. These properties enable a system to be constructed that allows the transmission of information from multiple points along the single fiber that may be related to data transmission, sensing information, or a combination of both.
There is provided by this invention a Sagnac interferometer based sensing system that allows the measurement of the amplitude of a time varying event along a single fiber and with suitably placed reflectors, localization of the event.
The system includes a light source that is generally broadband with a low coherence length. The light source injects light into a fiber beamsplitter that is used to generate counterpropagating light beams in a Sagnac loop. The loop includes two facing fiber beamsplitters connected together at both inner legs, with one of the output legs of the second beamsplitter being connected to a single fiber that is the sensing leg of the system. The sensing leg has the ability to measure time varying signals such as acoustics or vibrations. By using a controlled device to induce time varying changes in optical pathlength on the fiber, data may be sent. Placing multiple colored reflectors in line allows environmental effects to be localized and data to be transmitted from multiple points.
Therefore, it is an object of the present invention to provide a single fiber Sagnac system that can be used to measure time varying events.
Another object is to enable the determination of the position of a time varying event along the single fiber line.
Another object is simultaneously measure the amplitude and location of a time varying event allowing information necessary for classification.
Another object of the invention is to allow the transmission of data via the simple attachment of a phase modulator to a sensing leg.
Another object of the invention is to allow the transmission of data from multiple points along a single fiber.
These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detailed specification including the drawings wherein: